Sirens blasting from Aberdeen Proving Ground tomorrow will be the first clear signal that someone is preparing for the unlikely event of a major accident involving the Army post's large and potentially dangerous stockpile of mustard agent in a field along the Bush River.

After the tests, set for 1 p.m. to 4 p.m., 41 sirens are to be installed by March in Harford, Baltimore and Kent counties.

"They will be the first visible thing to the public," said Robert Dickover, a Baltimore County emergency-preparedness official.

For four years, however, governments around the proving ground quietly have spent millions on advanced communications equipment, road projects to enhance evacuations and plans to make schools safe from such an accident, the odds against which are put at 100 million to one.

However remote the chances, the prospect of an accident involving the proving ground's estimated 1,500 tons of mustard agent has generated a rare windfall of federal money for the three counties.

The counties and the Maryland Emergency Management Agency have been authorized to spend more than $8 million in Army money, and the jurisdictions have requested $4 million more for the federal budget year that begins Oct. 1. It is expected that millions more will be spent.

On Wednesday, state and local officials will participate in the first of three emergency exercises. The second is set for November and the third, the first full-scale exercise, for April.

Nationally, nearly $190 million has been spent on the emergency program. In addition to the proving ground, there are seven other U.S. sites where aging, lethal warfare chemicals are kept.

Some members of Congress have criticized the slow pace of the emergency program. In Harford, for example, officials have waited two years to be told which types of protective suits and other gear are needed by firefighters, sheriff's deputies and others who would respond to an accident.

All the planning, training and hardware paid for by the Chemical Stockpile Emergency Preparedness Program (CSEPP) helps the counties prepare for any major accident or disaster -- from chemical spills on Interstate 95 in Baltimore or Harford counties, to accidents at fertilizer plants in Kent, to hurricanes or major storms in the upper Chesapeake Bay region.

"There's stuff going up and down the roads and rails every day that makes mustard agent look like Sprite," said James W. Terrell, chief of Harford County's Department of Emergency Operations.

Political battles have raged locally and nationally since the mid-1980s, when Congress told the Army to dispose of its mustard and nerve agents at the proving ground and the seven other U.S. sites. The Army chose to build huge incinerators.

As the fight over the safest way to dispose of the chemicals continues, and the stockpiles remain, Congress also has directed the Army to help neighboring communities prepare for accidents.

Mustard agent, a syrupy liquid often incorrectly called mustard "gas," blisters the skin and burns the respiratory system. In lower doses, it can cause chronic injuries; in higher doses, it can be fatal. It causes cancer in laboratory animals.

Experts say a catastrophic event -- a plane crashing into the stockpile and a fire burning uncontrolled for at least 30 minutes -- would be required to present any significant danger to people around the proving ground. Intense heat would be needed for the chemical to get into the air and be carried beyond the installation's borders, they say.

Though the chemical has been stored at the proving ground since World War II, the Army didn't publicly acknowledge the stockpile's existence until the mid-1980s. Consequently, an emergency-preparedness effort involving neighboring communities could not begin until the stockpile was declassified.

CSEPP itself, though it is not directly related to any incineration plan or other disposal method, has generated controversy.

The Kent County commissioners, saying they were "vehemently opposed" to any plan to incinerate the mustard agent, withdraw from the emergency program last year. Taking Army money for emergency planning would constitute indirect support for the plan to build a $438 incinerator, they said.

Within three months, however, Kent was back in the emergency program. Congress told the Army to take a second look at possibly cheaper and safer ways to dispose of the chemicals at the proving ground and other sites where lesser amounts are stored or where there are large civilian populations.

An estimated 300,000 people live within 15 miles of the proving ground's stockpile. That is the area in which the sirens will be installed and where residents could be asked to take action during an emergency.